cxlvi THE VOYAGE OF H.M.S. CHALLENGER. 



CHOROLOGIOAL SECTION. 



Chapter IX.— GEOGRAPHICAL DISTRIBUTION. 



(§§ 226-240.) 



226. Universal Marine Distribution. — Radiolaria occur in all the seas of the world, 

 in all climatic zones and at all depths. Probably under normal conditions they always 

 float freely in the water, whether their usual position be at the surface (pelagic), or at a 

 certain depth (zonarial), or near to the bottom of the sea (abyssal). This apj^ears both 

 from numerous direct observations, as well as from conclusions which may be drawn from 

 their organisation (and especially their promorphology) regarding their floatinof life 

 (compare §§ 40-50, 219, 220). Hitherto no observation has been recorded, which justifies 

 the assumption that Radiolaria live anywhere upon the bottom of the sea (on stones, Algfe, 

 or other firm substances), either sessile or creeping. They perform the latter action, 

 however, when they fall accidentally upon a firm basis or are accidentally placed uj3on it, 

 l^ut they seem normally always to float freely in the water with pseudopodia radiating in 

 all directions. Active free-swimming movements are only met with in the case of the 

 flagellate zoospores (§ 142). The development of Radiolaria in large masses is very 

 remarkable (see note A), and in many parts of the ocean is so great that they play an 

 important part in the economy of marine life, especially as food for other pelagic and 

 abyssal animals (see note B). Medium salinity of the water seems to be most favourable 

 to their development in masses, although it is not unknown in seas of high and low 

 salinity (see note C). There are no Radiolaria in fresh water (see note D). 



A. The development of Eadiolaria takes place in many parts of the ocean in astonishingly large 

 masses on the surface, in different strata, and near the bottom. The Collodaria (and especially 

 the Splnerozoida) often cover the surface of the sea in millions, and form a shining layer, phosphor- 

 escent in the dark like the Noctiliu:w, as I observed in 1859 in the Strait of Messina, in 1866 at the 

 Canaries, and in 1881 in the Indian Ocean. Similar masses of Sphccrozoum and Acanthometron 

 were seen by Johannes Miiller on the French and Ligurian coasts (L. N. 12), and John Murray 

 found another in the Gulf Stream, off the Fwroe Islands, from the surface to a depth of 600 fathoms; 

 considerable masses of large PHiEODAEiA live there also. 



B. The alimentary canal of Medusie, Salpte, Crustacea, Pteropoda, and many other pelagic 

 animals is a rich field for the discovery of Radiolaria, and many of the species hereinafter described 

 are from such sources. Fossil coprolites too (e.g., those from the Jura) often contain many 

 Polycystina. 



C. Some AcANTHARiA (A c a n t h o m e t r a) and PHiEODAEiA (species of Mcsocena and Bictyocha) 



